Malaria currently threatens nearly half the world's population and is responsible for an estimated 500 million clinical cases and one to three million deaths a year. Although malaria can be controlled by public health interventions, throughout the tropics it is treated primarily through chemotherapy. P. falciparum populations preserved in approximately 11,000 archived human sera collected from the 1950s through the 1980s will be analyzed to retrospectively document the genetic changes that resulted in chloroquine (CQ) treatment failure in the Pacific, one of the five independent foci of CQ resistance evolution. Three P. falciparum loci from each of the sera samples will be amplified via PCR. Preliminary studies suggest that approximately 10% of the samples will contain amplifiable P. falciparum DNA. The accumulation over four decades of the nine previously identified amino acid changes in pfcrt that confer CQ resistance will be documented. Amino acid changes in the pfmdrl gene, thought to stabilize the mutations in pfcrt will also be tracked. Sequence variation of the mitochondrial DNA of the parasites will also be assessed to explicitly determine their geographic origins to evaluate the possible influence of human facilitated P. falciparum gene flow on the evolution of CQ resistance. The NIH-NINDS-LCNSS Serum Archive housed at Binghamton University preserves P. falciparum populations that span the critical periods of drug tolerance and resistance evolution and the subsequent spread of resistance. The earliest samples were collected from extremely isolated populations in the 1950s that had little contact with other human groups and minimal exposure to CQ or other antimalarials. These parasite populations represent "natural" diversity unaffected by selective sweeps resulting from drug pressure. The archive contains extensive samples from the 1960s that are expected to contain parasites evolving tolerance to CQ prior to the appearance of CQ treatment failure in the early 1970s. The samples from the 1970s should contain the earliest CQ resistant parasites and those from the late 1970s and early 1980s will provide insight on the temporal and spatial sweep of CQ resistance throughout the region. This study will document the genetic evolution of CQ resistance by P. falciparum in the Pacific and thus, provide an in vivo model of the evolution of drug tolerance and resistance over four decades to forecast the evolution of resistance to other antimalarials and their combinations.